This invention relates to a system for controlling the discharge temperature of a high pressure stage of a multi-stage centrifugal compression unit of the type utilizing relatively low pressure refrigerant discharged from a low pressure stage to satisfy a cooling load and relatively high pressure refrigerant discharged from the high pressure stage to satisfy a heating load.
In air conditioning large buildings, and in other similar applications, it is often necessary to provide cooling and heating simultaneously. For example, the outer portion of the building will require heating in cold weather to compensate for transmission losses through walls and windows; while the inner portion or core of the building will require cooling to compensate for the heat buildup in the center or unexposed portions of the building. During warm weather the heating requirement will diminish and the entire building may require cooling. In order to provide both heating and cooling, refrigeration systems have been developed which utilize a conventional centrifugal compressor, condensor and evaporator to promote cooling and which employ a second or high pressure condenser to provide heating. The heating condenser receives hot refrigerant vapor from a second or higher pressure stage of a multiple stage centrifugal compression unit. The hot vapor is condensed in the heating condenser, thereby heating water or other suitable medium to a temperature sufficient for use in heating the building.
As is fairly apparent, the heating load on the high pressure stage will not remain constant, but rather will vary in accordance with changes in the ambient temperature. However, irrespective of such changes in the heating load, it is necessary to maintain a continuous flow of refrigerant through the high pressure stage to prevent such stage from overheating. If the refrigerant furnished to the high pressure stage at relatively low heating load conditions is supplied at normal suction conditions for said stage, i.e. at the low pressure stage discharge pressure, a substantial weight flow of vapor is required, with the stage using a relatively large quantity of power to further compress the refrigerant while no useful work is being accomplished.
Although it is known that reducing the mass flow through the compressor stage will reduce the power requirements thereof, a mere reduction in such flow wthout a simutaneous decrease in the pressure differential across the stage, will cause the stage to operate near surge conditions. As is well recognized, it is undesirable to operate a centrifugal compressor at or near surge conditions due to the high discharge temperatures and mechanical vibrations that are generated at such times. If the refrigerant vapor is supplied at a much lower pressure when the heating load on the high pressure stage is reduced, the mass or weight flow of refrigerant will be concomitantly reduced thereby decreasing the consumption of wasted energy. Further, by lowering the pressure differential across the stage, while simultaneously lowering the weight flow therethrough, the compressor will be prevented from operating at or near surge conditions.